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Sci Rep ; 5: 15088, 2015 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-26462615


We describe an experimental method to probe the adsorption of water at the surface of isolated, substrate-free TiO2 nanoparticles (NPs) based on soft X-ray spectroscopy in the gas phase using synchrotron radiation. To understand the interfacial properties between water and TiO2 surface, a water shell was adsorbed at the surface of TiO2 NPs. We used two different ways to control the hydration level of the NPs: in the first scheme, initially solvated NPs were dried and in the second one, dry NPs generated thanks to a commercial aerosol generator were exposed to water vapor. XPS was used to identify the signature of the water layer shell on the surface of the free TiO2 NPs and made it possible to follow the evolution of their hydration state. The results obtained allow the establishment of a qualitative determination of isolated NPs' surface states, as well as to unravel water adsorption mechanisms. This method appears to be a unique approach to investigate the interface between an isolated nano-object and a solvent over-layer, paving the way towards new investigation methods in heterogeneous catalysis on nanomaterials.

ACS Appl Mater Interfaces ; 7(12): 6637-44, 2015 Apr 01.
Artigo em Inglês | MEDLINE | ID: mdl-25761636


Carbon-covered silicon nanoparticles (Si@C) were synthesized for the first time by a one-step continuous process in a novel two stages laser pyrolysis reactor. Crystallized silicon cores formed in a first stage were covered in the second stage by a continuous shell mainly consisting in low organized sp(2) carbon. At the Si/C interface silicon carbide is absent. Moreover, the presence of silicon oxide is reduced compared to materials synthesized in several steps, allowing the use of such material as promising anode material in lithium-ion batteries (LIB). Auger Electron Spectroscopy (AES) analysis of the samples at both SiKLL and SiLVV edges proved the uniformity of the carbon coating. Cyclic voltammetry was used to compare the stability of Si and Si@C active materials. In half-cell configuration, Si@C exhibits a high and stable capacity of 2400 mAh g(-1) at C/10 and up to 500 mAh g(-1) over 500 cycles at 2C. The retention of the capacity is attributed to the protective effect of the carbon shell, which avoids direct contact between the silicon surface and the electrolyte.

J Phys Chem Lett ; 5(19): 3399-403, 2014 Oct 02.
Artigo em Inglês | MEDLINE | ID: mdl-26278452


X-ray photoelectron spectroscopy (XPS) is a very efficient and still progressing surface analysis technique. However, when applied to nano-objects, this technique faces drawbacks due to interactions with the substrate and sample charging effects. We present a new experimental approach to XPS based on coupling soft X-ray synchrotron radiation with an in-vacuum beam of free nanoparticles, focused by an aerodynamic lens system. The structure of the Si/SiO2 interface was probed without any substrate interaction or charging effects for silicon nanocrystals previously oxidized in ambient air. Complete characterization of the surface was obtained. The Si 2p core level spectrum reveals a nonabrupt interface.